Fatty acid synthetase, obtained from mammalian tissues, was shown to catalyze a few novel reactions, e.g., the reduction of acetoacetyl-CoA and of trans-crotonyl-CoA at micromolar concentrations and to form substantial quantities of butyrate in addition to long chain acids. In the coming year our investigations will be directed towards: a) evaluating the physiological significance, if any, of these reactions; and b) understanding the mechanism by which free coenzyme A and palmitoyl-CoA regulates the rate of fatty acid synthesis.